HandBrake, doesn't compress them, it reencodes them, slightly different. Anyway, for your purpose, H265 software on CPU should get the best results. try with a shorter video, with different settings to have an idea of the end result (you are gonna lose quality in one way or another, how much and where depends on you). Then simply let it go overnight, depending on how many files you have, your hardware and your settings, it can easily be 10s of hours. Get an idea of how much time it takes and do it in batches if you are gonna need to use the pc during the day for example. You could limit the program to only use a certain amount of cores so you have some left for you daily use, but if you game for example, its a no go. encode during the night, play during the day